If you can wait a while, I'll post a picture on how to read codes without a scan tool. If you can't wait, you can get a tool at an autoparts store. Don't get anything fancy. Just one of the ones that run about $30. It basically a couple switches with a light and a beeper on it. It pretty much does what you can do with just jumpering a wire or two.
Failing sensors can cause those problems, vacuum leaks, fuel injectors, fuel pump, relays, etc. Code reading is a good way to find out if any sensors are failing. If no codes are there, then it's a long tedious process of elimination.
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351W 89 Mustang GT Convertible